Devour nuts, entertained by those running the Grand National courses (4,8)

'grand national courses' is the definition.
'hors d'oeuvres' can be an answer for 'courses' (hors d'oeuvre is a kind of course). I am not certain of the 'grand national' bit.

'devour nuts entertained by those running' is the wordplay.
'nuts' is an anagram indicator (nuts can mean crazy).
'entertained by' means one lot of letters goes inside another (inserted letters are hosted or entertained).
'those running' becomes 'horses' (I can't justify this - if you can you should give a lot more credence to this answer).
'devour' with letters rearranged gives 'doeuvr'.
'doeuvr' going into 'horses' is 'HORS DOEUVRES'.

'the' acts as a link.

Can you help me to learn more?